The present invention relates to a device in an electrostatic precipitator for the suspending, controlling and rapping of one or more collecting electrodes arranged essentially vertically in one or more substantially parallel rows and comprising for each row a substantially horizontally oriented carrier element to which the upper ends of the collecting electrodes are attached, connecting elements which connect the carrier element to the casing of the electrostatic precipitator, control means for controlling the motion of each row of collecting electrodes in the transverse and/or longitudinal direction of the electrostatic precipitator, and a rapping mechanism for rapping the collecting electrodes of each row and comprising a rapping means, such as a rapping hammer, and an anvil connected to the carrier element.
Electrostatic precipitators are usually composed of a plurality of successive precipitation fields through which dust-laden gas is passed to be cleaned. Each of the precipitation fields is divided into a plurality of parallel gas passages by means of a plurality of juxtaposed, earthed collecting electrodes vertically arranged in rows . These are usually in the form of rectangular, substantially plate-shaped, sectional metal sheets. Vertical discharge electrodes, to which a negative voltage is supplied, are arranged in each gas passage. Owing to corona discharges at the discharge electrodes, the dust-laden gas is ionised in the electric field in the gas passages. The negative ions are attracted by the collecting electrodes and, while moving towards these, collide with the dust particles of the gas, which are then charged and separated from the gas by being attracted by the closest collecting electrode, on which they are deposited and build up a dust layer. At regular intervals, the dust layer is loosened, by vibrations, from the collecting electrodes by these being mechanically affected by means of a rapping mechanism. The dust particles then fall into a collecting hopper associated with the respective precipitation fields.
The rapping mechanism can either be arranged at the top or at the bottom of each row of collecting electrodes and usually comprises a rapping hammer acting in a vertical plane about a rotary shaft which extends horizontally in the transverse direction of the electrostatic precipitator and to which the rapping hammer thus is connected, and an anvil. The rapping force for cleaning each row of collecting electrodes is generated by the rapping hammer giving a horizontally directed rap via the anvil to a rapping means cooperating with the collecting electrodes, whereby a horizontal motion is imparted to the rapping means. This motion is transferred to the collecting electrodes in the form of a shock wave which spreads across these. Thus, the dust layer accumulated on the collecting electrodes is loosened.
A greater or smaller amount of the rapping force is absorbed by the suspension of the collecting electrodes during the cleaning operation. The amount of absorbed rapping force depends on the manner of connecting the collecting electrodes to the suspension and on the manner in which the suspension is connected to the casing of the electrostatic precipitator.
The collecting electrodes are usually controlled in the longitudinal as well as in the transverse direction of the electrostatic precipitator so as to be kept in exact positions and, thus, prevent flashovers.
There have been suggested various solutions of a device in an electrostatic precipitator for the suspending, controlling and rapping of its collecting electrodes.
For instance, EP 0 584 880 discloses a device for cleaning collecting electrodes in an electrostatic precipitator by rapping, the collecting electrodes being vertically arranged, successively in parallel rows. The collecting electrodes of each rows are, at their upper ends, attached between a pair of horizontally arranged longitudinal rapping beams and suspended from these, said beams thus also serving as carrier elements for the collecting electrodes. The rapping beams rest freely on a pair of supporting beams, between which the upper ends of the collecting electrodes are arranged. The pair of supporting beams rests, in turn, on a frame pertaining to the casing of the electrostatic precipitator. The pair of rapping beams thus is arranged so as to be able to slide, during rapping, horizontally on the supporting beams in the longitudinal direction of the electrostatic precipitator. During cleaning by rapping, the rapping force is transferred in the form of a shock wave by means of a rapping hammer to all the collecting electrodes of a row via an anvil, which is fixedly mounted on the pair of rapping beams of each row. On each side of the ends of the pair of rapping beams, laminated springs are mounted in the casing and cooperate with transverse so-called stop pins arranged at each end of the pair of rapping beams. A spring-back motion of the pair of rapping beams to its original position is permitted in connection with the rapping before the anvil is again hit by the rapping hammer. As a result, the laminated springs control the row of collecting electrodes in the longitudinal direction of the electrostatic precipitator. The laminated springs also serve as control means in the transverse direction of the electrostatic precipitator. A drawback of the thus disclosed device is that a relatively great amount of the rapping force is absorbed by the casing during rapping and, thus, is lost. Besides, for each row of collecting electrodes, the device consists of a large number of components, which makes it mechanically complicated.
The mounting of the respective rows is carried out in such a manner that the upper ends of the collecting electrodes are first inserted between the pair of the supporting beams and are temporarily mounted therein by means of bolts. Each rapping beam is then arranged on both sides of the upwardly extending ends of the collecting electrodes and is mounted therein by means of bolts, whereupon the bolts in the pair of supporting beams are removed. The mounting procedure accomplished in this manner thus is complicated and time-consuming. The mounting of each row will probably also take place inside the electrostatic precipitator, which is a drawback compared with the technique of mounting the row outside the electrostatic precipitator so as to be inserted afterwards.
One object of the present invention thus is, in view of that stated above, to provide a new and improved device in an electrostatic precipitator for suspending, controlling and rapping its collecting electrodes, said device allowing a more efficient use of the generated rapping force when cleaning the collecting electrodes. A further object of the present invention is to provide a device for suspending, controlling and rapping, which is mechanically simpler and, consequently, easier to mount.
These objects are achieved by a device which is of the type stated by way of introduction and which according to the present invention is characterised in that the carrier element of each row is suspended separately by means of connecting elements, which as stated above connect the carrier element to the casing of the electrostatic precipitator, thereby permitting a minimum horizontal pivoting motion restricted to each collecting electrode row and occurring in the longitudinal direction of the electrostatic precipitator. The pivoting motion arising in rapping occurs owing to the separate suspension and thus fully independently of the remaining rows. By a restricted, minimum pivot motion is meant a pivoting motion which is less than about 5 mm of the carrier element in its longitudinal direction.
According to the present invention, the connecting elements of the carrier element preferably comprise two elongate rods, each making an angle .alpha., relative to a vertical axis extending through each rod, in the range of about 5-35.degree., especially in the range of about 5-15.degree., in such a manner that said rods are positioned substantially in the plane of the row such that the distance between their lower ends is smaller than between their upper ends. In consequence of the carrier element being separately suspended in the above-mentioned fashion, the row of collecting electrodes will be self-centred after rapping, i.e. the row will immediately return to its correct original position.
The smaller the angle a the more movable the actual suspension, which means that a greater amount of the rapping force, relatively seen, will be absorbed by the casing, which results in poorer cleaning. On the other hand, the greater the angle .alpha., the more unresilient the suspension, which also results in poorer cleaning. To achieve optimum cleaning, i.e. to transfer as great an amount as possible of the rapping force to the collecting electrodes, the angle is adjusted according to the present invention so that as small an amount as possible of the rapping force is absorbed by the casing. Moreover, the angle .alpha. should be adjusted so as to prevent too great a pivoting motion of the collecting electrodes. By such suspension, which is mechanically simple and easy to mount, a more efficient cleaning of the collecting electrodes is provided.
According to the present invention, the control means of the device consist of upper control elements cooperating with the carrier element, and/or lower control elements cooperating with the lower ends of the collecting electrodes. The upper control elements preferably consist of upper cam control elements cooperating with the end portions of the carrier element. Advantageously, the cam control elements extend substantially horizontally in the transverse direction of the electrostatic precipitator through a plurality of rows and thus prevent each row of collecting electrodes from moving in said direction. The upper control elements also serve as control means in the longitudinal direction of the electrostatic precipitator, especially in case of small angles .alpha.. By means of these upper control elements, which are mechanically simple and easy to mount, efficient control is achieved.
To further improve the control, in addition to the control provided by means of the upper control elements, lower control elements are preferably arranged at the bottom of each row and cooperate, as stated above, with the lower ends of the collecting electrodes.
According to a preferred embodiment of the inventive device, the lower ends of the collecting electrodes are loosely controlled by the lower control elements on the one hand in the transverse direction of the electrostatic precipitator by means of a longitudinally oriented, horizontal pair of rods, between which the lower ends of the collecting electrodes are arranged and, on the other hand, in the longitudinal direction of the electrostatic precipitator by means of spacer members arranged before, after and/or between the collecting electrodes.
A further alternative preferred embodiment of the inventive device is characterised in that the lower ends of the collecting electrodes are firmly controlled by the lower control elements in the transverse as well as longitudinal direction of the electric precipitator by means of longitudinally oriented, horizontal rods, to which the lower ends of the collecting electrodes are attached, preferably by means of bolts or rivets.
A common feature of the lower control is that each row of collecting electrodes is controlled in the transverse direction of the electrostatic precipitator by means of transversely oriented lower cam control elements arranged at the ends of said rods. The cam control elements advantageously extend substantially vertically in the transverse direction of the electrostatic precipitator through a number of rows and thus prevent the respective rows of collecting electrodes from moving in said direction. By means of these lower control elements, which are mechanically simple and easy to mount, efficient control is achieved. The lower and the upper control according to the invention is further designed with regard to thermal expansion.
According to the present invention, the anvil of the rapping mechanism is preferably directly fixedly mounted on the upper side of the carrier element and comprises a rapping shoulder, which is the rapping point of the rapping means, which is arranged in such a manner that the rapping point is located above the horizontal centre line of the carrier element. According to an alternative embodiment, the anvil can also extend across at least two rows of collecting electrodes in the transverse direction of the electrostatic precipitator. In this case, the anvil, which consists of e.g. a flat iron bar, is also directly fixedly mounted on the upper side of the carrier element of each row.
The rapping means of the rapping mechanism preferably acts in a vertical plane about a rotary shaft connected thereto and extending horizontally in the transverse direction of the electric precipitator, the rapping means giving the anvil a horizontally directed rap during rapping. In dependence on the embodiment, the rap is given to the anvil either directly or indirectly via the above-mentioned rapping shoulder.
A general advantage of the inventive device is that each row of collecting electrodes can be mounted outside the electrostatic precipitator and then be inserted.